Generally, a gate valve gate has an interiorly threaded opening which receives an exteriorly threaded rod. The rod is helically rotated and/or axially moved in order to close and open the gate. Handwheels are used to move the rods manually; pressurized fluid powered, piston-operated actuators are used for moving the rods automatically and/or from a remote location.
Now-conventional gate-to-threaded operator connections are shown in these prior U.S. patents: U.S. Pat. No. 3,349,789, Crain et al., Oct. 31, 1967 U.S. Pat. No. 3,789,875, McGee, Feb. 5, 1974 U.S. Pat. No. 3,990,679, Boitnott, Nov. 9, 1976
Others have previously proposed the use of an intermediate bushing for threadably connecting a gate with a valve stem. Examples are shown in the following prior U.S. patents: U.S. Pat. No. 1,243,736, Hibbard, Oct. 23, 1917 U.S. Pat. No. 2,192,331, Schaefer, Mar. 5, 1940 U.S. Pat. No. 2,276,620, Harrington, Mar. 17, 1942 U.S. Pat. No. 3,662,778, Leopold, Jr. et al., May 16, 1972 U.S. Pat. No. 3,788,600, Allen, Jan. 29, 1974
The Allen patent shows the stem screwed directly into the gate with a splined thrust bearing arrangement in the bonnet. The purpose of the splined arrangement is to allow longitudinal movement for a safety seal operation. The axial force is transmitted directly to the gate via the threads on the stems.
The Harrington patent shows a drive nut connected to a toggle liner which engages a finger member which in turn is bolted to the wedge type gates. The purpose of this arrangement appears to be to apply axial force to one gate member only and to spread the gates laterally.
The Leopold patent shows a drive bushing having lugs on the outer surface to fit mating recesses on the gate member. The axial force is transmitted by these lugs to the gate member.
The Hibbard patent shows a ball socket type of drive bushing but it is connected to a threaded member which raises and lowers the gate member. The parts 15, 16 and 17 fit into an essentially square recess in the gate member and would appear to have the same effect as a square nut and pocket.
The Schaefer patent shows a tee-shaped nut which runs up and down on a threaded stem and a keyway in the valve body to guide the tongues of the nut in its up and down travel. There would be no axial force transmitted to the gate members. However, it would appear that precise fit, alignment, tolerances, friction, wear, buckling, binding of stem, and accumulation of debris in these keyways would make keyways undesirable.
In the prior art, particularly when the stem is threaded directly into the gate as in Allen or having lugs as per Leopold or having an essentially square nut and pocket as per Hibbard, the axial load concentration is highest near the center of rotation when a given amount of torque is applied. The load concentration is reduced when applying the same amount of torque at a point further from the center. The Allen, Leopold and Hibbard patents have this load concentration in the threads, lugs and square nut design. The part of the drive nut designed to prevent rotation of the nut will tend to shear off or become rounded as in the case of lugs or square nuts. This may also cause deformation and stress build-up in the portion of the gate that mates with the lugs or nut.